Polymer matrix composite structures can be modified to form electromagnetic interference (EMI) shielding materials. For example, EMI shielding in the form of metal-coated fibers forming a discontinuous phase in a continuous phase matrix material is disclosed in U.S. Pat. No. 4,935,296. Radar absorbing materials are described in U.S. Pat. No. 5,661,484. Electrically conductive laminates, conductive coatings, conductive adhesives, conductive inks, conductive gaskets and conductive caulking and sealing compounds using metal coated fibers are disclosed in U.S. Pat. No. 4,983,456. Electrically conductive bulk, granular or nodular molding compounds comprising a multi-component filler system of particulate carbon black, graphite and metal particles are disclosed in U.S. Pat. No. 4,474,685.
Polymer matrix composites can be been manufactured by several techniques, such as preimpregnated fabric hand lay-up (so called prepreg) techniques and by resin transfer molding (RTM) techniques. Prepreg techniques generally include depositing resin on structural supports, such as a cloth, fabric or fiberglass, stacking the structural supports, and heating the stacked structural supports in an external heat source, such as an oven, an autoclave, or a heat lamp. Similarly, RTM techniques assemble dry structural reinforcements in a specifically designed tool, pump resin using a positive displacement pump, and apply pressure and/or temperature to form the polymer matrix composite structure.
With both prepreg and RTM methods, the resin flows during heating to penetrate throughout the structure and subsequently cures, thereby forming a unitary polymer matrix composite structure. However, the flow of the resin can also cause portions of the structure in the structural support to shift position, thereby forming defects and/or voids within the polymer matrix composite. In addition, the flow of resin can be incomplete in that resin does not penetrate into certain areas of the structural support and/or penetrates nonuniformly. Incomplete resin flow can also contribute to the formation of defects and/or voids within the polymer matrix composite.